Thermoplastic polymers are well known, among other things, to offer the advantages of low density, good stiffness, chemical resistance, possess an ability to be formed into various shapes and relatively low cost. Unfortunately, this class of polymers is also known to possess relatively high thermal expansion values. Values found in the literature for common thermoplastics such as polyethylene, polypropylene, PVC, polyester, and nylon typically range from 50*10−6 m/(m*° C.) to 250*10−6 m/(m*° C.). Those of ordinary skill in the art of plastics fully appreciate the importance of using materials with low thermal expansion coefficients when designing a plastic article for applications that involve a change of temperature.
Conventional efforts to lower the thermal expansion coefficients of thermoplastics involve making composites by the incorporation of various fillers. Fillers can include mineral fillers such as calcium carbonate, talc, clay, volcanic ash or various nanoparticles. Fillers can also include organic fillers such as wood flour, rice hulls, or corn byproducts. It is also known to employ fibers such a carbon fibers, various polymer fibers, cellulose fibers, or glass fibers and combine them with polymer melt processing techniques to form thermoplastic composites. Such fibers may be incorporated as loose fibers or orientated fibers in the polymer or as woven or non-woven sheets. The woven or non-woven sheets are often first made into relatively thin webs of a low basis weight that have thermoplastic or thermoset polymers incorporated into them. They are then typically applied as layers to ultimately create a multilayered substrate. Unfortunately, the addition of excessive filler or fibers in an effort to achieve lower thermal expansion coefficients can compromise other properties of thermoplastic composites. For example, the resulting composite may undesirably exhibit the reduction of one or more of its weight, overall flexibility, cost, or impact strength. It can also become very difficult to mix high amounts of fillers into thermoplastics.
The thermal expansion coefficient of thermoplastic composite materials is very dependent on the thermoplastic resin being used. Thermoplastic resins such as polyethylene and polypropylene, which have high thermal expansion coefficients, are more difficult to modify into thermoplastic composites having a very low thermal expansion coefficient. Thermoplastics such as PVC and polyester have lower thermal expansion coefficients than polyolefins, however there remains a need in the marketplace for thermoplastic composites having even lower thermal expansion coefficients than presently available.